Ultrasonic flow meter systems are known for measuring the rate of fluid (e.g., gas or liquid) flow within a conduit such as a pipe. In one particular system, two transducers are disposed on the exterior of the conduit at an oblique angle to each other and are commonly referred to as the upstream transducer and the downstream transducer. The rate of fluid flow through the conduit is determined by first transmitting a pulse from the upstream transducer to the downstream transducer. Next, the downstream transducer transmits a pulse to the upstream transducer. The transit time of the pulse transmitted from the upstream transducer to the downstream transducer is less than that of the pulse transmitted in the reverse direction and the fluid flow rate can be determined (calculated) based on the difference in the measured transit times of the two pulses. Those skilled in the art know that the transducers can be clamped on the exterior of the conduit or can be inserted through the wall of the conduit (e.g., “wetted transducers”).
Ultrasonic systems may be used to measure the flow of many different types and densities of liquid or gas and may be used with different types of conduits in varying degrees of condition. These widely varying environments can create different signal to noise ratios when one of the transducers transmits to the other transducer. For example, if the fluid flow to be measured is liquid, the signal to noise ratio will generally be high. If the fluid flow to be measured is gas, the signal to noise ratio will generally be lower. Depending on the signal to noise ratio, different techniques are employed to measure the transit time and time differential between the up and down transit times. For example, if the signal to noise ratio is high, one high resolution technique for measuring the transit time is the cross correlation technique. See U.S. Pat. No. 4,787,252 incorporated herein by this reference. But, if the signal to noise ratio is low, the high resolution method cross correlation technique may result in errors when the transit time is calculated resulting in an inaccurate flow rate determination. Thus, those skilled in the art may employ a different technique for measuring the transit time. One example of a technique for measuring the transit time where the signal to noise ratio is low is the integrated threshold technique disclosed in U.S. Pat. No. 4,538,469 also incorporated herein by this reference. Although the integrated threshold technique doesn't provide as high resolution as the cross correlation technique, it is more robust.
Moreover, the signal to noise ratio of any particular transducer arrangement may change over time if the conduit deteriorates, for example, the signal to noise ratio may decrease. Thus, an ultrasonic flow meter system controller (electronics) may be configured at the time of installation to employ the cross correlation technique because the signal to noise ratio is high. But, later in time if the signal to noise ratio is reduced, the cross correlation technique will no longer provide an accurate flow rate determination.
In the prior art, it was often necessary to have a technician or engineer set up an ultrasonic flow meter after evaluating the flow measurement site by connecting an oscilloscope to the meter (or controller) to determine the signal to noise ratio before a particular technique for calculating the transit time of each transmitted pulse could be selected and the transit time measured. This practice is time consuming, expensive, and at times may not be accurate. Moreover, an oscilloscope is an expensive piece of equipment that takes expertise to operate.